For all of its peaceful aspiration, however, the U.N. Charter has been ineffective in constraining state military behavior as the political and technological circumstances of the world have evolved. The Security Council did not reach agreement on establishing an international bomber force. The Council has rarely invoked most of its other coercive powers. Of course, that is not because the world has experienced peace. Instead, the differing agendas of the permanent members of the Security Council have paralyzed the U.N., leaving it to nations to resolve disputes in other ways.54 While disputes have sometimes erupted into war, they have not yet escalated into a World War III. Some scholars believe that the “Long Peace” of the postwar world emerged from the balance between the U.S. and the U.S.S.R., while others argue that nuclear weapons imposed caution on all the great powers.55 We cannot be sure that even that reprieve will endure, now that Pakistan, North Korea, and, perhaps others have acquired nuclear weapons.
Even as the prospect of general war has receded, other forms of hostilities short of all-out war have emerged. Formal declarations of war have become rare—the U.S. Congress last enacted a formal declaration of war in 1942 (against Bulgaria). Perhaps that is partly because the U.N. Charter seems to prohibit resorting to “war.” But formal “war” has also become less easy to define because hostilities often fall short of open armed conflict, and formal peace treaties rarely mark their end. While the Second World War ended with unconditional surrender, Cold War disagreements prevented any general or comprehensive peace treaties among all participants. In Korea, a “temporary” armistice remains in place to this day, as with Israel and its neighbors (except Egypt and Jordan). North Vietnam ended its war in a unilateral annexation without awaiting any other nation’s approval.
The lines between war and peace (or conflict and trust) have become even more blurred in recent years, as major conflicts have involved non-state actors or states acting in disguised ways. If the Russian army had launched tank columns and regular infantry against the Baltic states, it would almost certainly have triggered a NATO military response and even a conventional interstate war. But the Russian invasion of Crimea in 2014 was much more ambiguous. Moscow sent special agents to seize strong points, enlisted local support, and disguised the underlying aggression with claims about local consent to what quickly became a fait accompli.56 Russia has continued the challenge by supporting local “militia” in eastern Ukraine who resist the authorities in Kiev, ostensibly in the interests of regional autonomy. Military analysts call this mix of covert operations combined with limited conventional military force a hybrid war, which often does not provoke the sort of response that would meet a full-scale invasion.57
New forms of irregular warfare do not just benefit revisionist states such as Russia, even small groups can now wage hostilities that generate instability and even seize territory. ISIS emerged from terror groups that had challenged the government established in Baghdad after Saddam’s overthrow. Terrorism has become another way to exert pressure without risking a direct trial of arms against an organized army. In the hands of ISIS, terrorism took on some of the characteristics of guerilla warfare, using surprise attacks and terrorizing civilians to take and hold land, people, and resources.58 In Lebanon and Syria, outside states such as Iran have made the challenges even more difficult by supporting armed militias with arms, training, and even troops. A regular army can defeat guerillas, but that has usually required a long struggle. Great powers, like the French in Algeria by the late 1950s, often lose patience with a lengthy struggle.
Hybrid war and guerrilla or terrorist insurgencies present the same set of strategic challenges for nations. Hybrid war tactics discourage initial intervention of an outside power by making the challenge seem limited, partial, and small stakes. Terror and guerilla tactics discourage a major power from making a long-term commitment for fear of a slow drain in low-intensity combat. Terrorists and guerrillas disperse to prevent a government from focusing its forces in conventional battle, and they seek cover behind the civilian population. They target civilians, even their own supporters, to intimidate them into remaining on their side. They aim to goad a government’s regular forces into making wider attacks on civilians, which may have the effect of driving civilians away from the state. The power with the greatest military muscle is not necessarily the most capable contestant. If the only answer to such irregular tactics is “war,” then a major power may decide not to respond.
States have also developed their own means of coercing opponents with tactics short of conventional war, such as annoying or harassing adversaries through intermediaries, another accepted military tactic that the U.N. Charter does not apparently endorse. As early as the American Revolutionary War, the British stirred up frontier Indians to attack American settlers. An independent United States winked at pirate attacks on Spanish commerce as a way of assisting independence movements in Latin America. In the nineteenth century, states deployed even more active measures. When small countries defaulted on loan agreements or seized or abused their nationals, major powers were often unwilling to declare war, either because the injuries were small or because they feared intervention by other powers. They instead resorted to “gunboat diplomacy.” Demands for redress were backed by a show of naval force, sometimes by small-scale bombardment. Nineteenth-century scholars of international law called it “pacific reprisal.”59 It was not war but a form of retaliation for the sake of coercion.
The U.N. Charter authorizes states to use force in self-defense “if an armed attack occurs.” But we have something close to pacific reprisals in the current policy of drone missile attacks on terrorists not only in Afghanistan but also in Pakistan, Yemen, and Libya. While the United States claims that it is acting in “self-defense” when conducting these strikes, it cannot credibly claim it is repelling an ongoing attack. Such strikes are consistent with the U.N. Charter only if we expand the concept of self-defense to include anticipation of an attack, even one that may not be imminent. In other words, the United States might claim that anticipatory self-defense allows preemptive strikes when the probability of an attack is small, but the potential for destruction is high. Or the United States and its allies must admit that they are engaging in preventive war designed to nip challenges to international security in the bud, even when there is no immediate claim to self-defense.
We are not arguing that war between states is disappearing, as some utopian writers do. Great power competitions continue to plague international politics, and those rivalries can still break out into conflict. Nuclear weapons may reduce the scope of hostilities, but the limits imposed by the superpower balance have eroded with the disappearance of the Soviet Union. The world is returning to a less orderly state of affairs. If the lines between war and peace blurred during the Cold War, they have become even less distinct today. Nations rely even more on measures short of full-blown armed conflict to coerce each other. New forms of international actors, such as terrorist groups, use force at a level that does not provoke a full-scale military response. Technological advances may provide western nations with a broader spectrum of coercion to respond. The rules imposed by the U.N. Charter and AP I cannot meaningfully govern these changes.
Responding to these new facts of war with new military technologies will provoke objections from international lawyers. They insist on highly restrictive rules for defining legitimate targets in war, such as AP I’s prohibition on the targeting of civilian property.60 Legalists can also invoke the AP I requirement that even attacks on otherwise legitimate “military objectives” are unlawful, if they “may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects . . . which would be excessive in relation to the concrete and direct military advantage anticipated.”61
These requirements of distinction and proportionality may make sense in a conventional war in which the main aim is to defeat the enemy’s army on the battlefield. But they make less sense if the aim is to affect the political calculus of enemy leaders. That was surely what the drafters of the U.N. Charter had in mind when they authorized interruption of sea or radio communication or even bombing as an “urgent military measure.” These tools built upon the blockades and economic embargoes that the Allies had used to deadly effect in World Wars I and II. Such actions seek not to defeat an enemy’s